Is Women's 'Retirement Gap' Really a Pay Gap?

Much is made of women's lack of engagement in basic investing, including investing for retirement. Reasons given range from a lack of confidence to a congenital fear of risk to a simple lack of information—when women gather, they apparently don't talk about their financial futures.

But here's a reason that may be too simple to get much attention: It could be that women invest less than men because they don't have as much money.

A new report from the Employment Benefits Research Institute about participation in employer-based pensions notes that women are less likely to take advantage of employer-offered retirement plans. This is in keeping with past studies.

But EBRI lists several social factors, besides being female, that are associated with lower levels of participation: being "nonwhite," uneducated, unhealthy, underemployed will do it, as will working in certain occupations, like farming, fishing, forestry or service jobs.

Youth will also inhibit workers from stashing cashing away for old age, even when those young people make as much money as their high-earning elders—less than half of workers 21 to 24 years old who were making $75,000 or more took part in a retirement plan, according to EBRI; 72 percent of 45-54 year old making that much did. (Read more: Women Are Breadwinners? It's Complicated)

Excepting age then, EBRI's report suggests that a large part of what keeps a worker from joining a company retirement plan is a lack of spare cash after present-day bills are paid, or uncertainty about their cash flow. Of those fulltime workers most likely to be offered a pension plan, those making at least $75,000 participated at twice the rate as those making between $30,000 and $40,000.

There are other factors than salaries to consider, of course. Since pension plans invest most of workers' contributions into the stocks and bonds, participation is closely tied to the markets. "The downturns in the economy and stock market in 2008 and into 2009 showed a two-year decline in both the number and percentage of workers participating in an employment-based retirement plan," said EBRI.

Accordingly, says the report, "2010 and 2011 participation levels stabilized as the economy recovered," moving from 39.6 percent in 2009, to 39.8 percent in 2010, to 39.7 last year.

The numbers for women alone tell a slightly different story, however. Over the past few decades, women have been closing the gap with men in retirement-fund participation rates. "In 1987, males participated at a level just over 10 percentage points higher than females," EBRI noted, but this gap had almost closed by 2009, narrowing to less than a single percentage point.

Women's participation didn't snap back as fast from the recession, however, and they fell further behind men again in 2010 and 2011, after overall participation in retirement funds had begun to rise again. (Read more: Returning Vets Fear Financial Risk)

It's worth noting that these dynamics correspond closely to the earnings gap between men and women. According to a recent figures from the Bureau of Labor Statistics, women's pay as a ratio of men's pay rose from about 70 percent in 1987 to about 80 percent in 2009 (though they were never as close to closing the pay gap entirely as they were the pension gap).

Then, just as their participation in retirement funds was falling in the past two years, many groups of women also lost a little ground in pay.

These numbers fit almost too neatly to explain the phenomenon of the "investment gap" fully. And there are, no doubt, cultural differences between men and women that make women less prone to save for retirement. But if we want to know the true effect of those differences, we might have to pay women equally first.

If that ever happens, we might even find that women outdo men for planning and investing for their futures.

Take it from Craig Copeland, the author of the EBRI report, who cites some "revealing findings" in his conclusion: "The overall percentage of females participating in a plan was lower than that of males," he wrote. "Yet, when controlling for work status or earnings, the female participation level actually surpasses that of males."